Mechanical efficiency comparison — two-stroke vs four-stroke IC engines Compared at similar size and technology level, a two-stroke cycle engine typically exhibits __________ mechanical efficiency than a four-stroke cycle engine. Choose the most appropriate option.

Introduction / Context:
Mechanical efficiency is the ratio of brake power to indicated power and reflects internal mechanical losses. Two-stroke engines often differ from four-strokes in scavenging, lubrication, and port timing, which influence friction and pumping losses.

Given Data / Assumptions:

• Conventional crankcase-scavenged or loop-scavenged two-strokes.
• Comparable displacement and technology to a four-stroke peer.
• Typical operating speeds and loads.

Concept / Approach:
Two-strokes generally have higher specific friction and pumping work due to continuous scavenging, port timing that can increase short-circuit losses, and lubrication regimes that raise friction. Consequently, for similar conditions, their mechanical efficiency tends to be lower than that of well-designed four-strokes that separate intake and exhaust strokes and often benefit from pressure-charging only when desired.

Step-by-Step Solution:
Identify mechanical loss sources: bearings, piston/ring friction, pumping/scavenging work.Two-stroke continuous gas exchange increases pumping effort per cycle.Therefore brake power/indicated power ratio is typically lower in two-strokes.

Verification / Alternative check:
Empirical brake-specific fuel consumption and friction mean effective pressure comparisons show higher FMEP for two-strokes at similar speeds.

Why Other Options Are Wrong:
“higher” reverses observed trends. “approximately the same, always” ignores architecture differences. “indeterminate because friction is zero” is physically false.

Common Pitfalls:
Confusing thermal efficiency with mechanical efficiency; two metrics are distinct.

Final Answer:
lower